Hydraulic tensioners have been widely used to maintain tension in timing chains and timing belts which transmit rotation from the crankshaft to one or more camshafts of a vehicle engine. By maintaining proper tension, the tensioner prevents vibration of the chain or belt.
A conventional hydraulic tensioner includes a housing having a cylindrical hole, a plunger slidable in the cylindrical hole and protruding from an opening at the end thereof, and a plunger-biasing spring urging the plunger in the protruding direction. The housing and the plunger together form a high pressure oil chamber, and a check valve mechanism, comprising a metal cylinder, is fitted into the cylindrical hole of the housing to permit entry of oil into the high pressure oil chamber, but prevent reverse flow of oil out of the chamber. The check valve mechanism is pre-assembled before being fitted into the cylindrical hole of the housing, and comprises cylindrical element providing an oil passage and having a ball seat formed thereon, a check ball, a coil spring, and a lid. An example of a conventional check valve mechanism is described in Japanese Laid-Open Patent Publication No. 2002-206603 (page 1, FIG. 1).
High machining accuracy is required for production of the conventional check valve mechanism, and assembly of a hydraulic tensioner incorporating the conventional check valve mechanism is difficult.
To address these problems, I have developed the hydraulic tensioner described in my Japanese patent application No. 2003-187277 and depicted in FIGS. 6-7(C) of this application. In this hydraulic tensioner 500, the check valve unit 540 comprises a synthetic resin ball guide 542, a ball seat 544 attached to the ball guide, a check ball 541 disposed in opposed relationship to the valve seat and guided by the ball guide 542, a check ball-biasing spring 545, which biases the check ball 541 toward the valve seat, and a disc-shaped retainer 543, which limits the extent to which the check ball 541 can move away from the ball seat 544. As shown in FIGS. 7(A), 7(B) and 7(C), two protrusions 542a are provided on opposite sides of the center of the ball guide 542 on the high pressure chamber side thereof. An outer circumferential tongue 542b is provided on the ball guide for pressing against the wall of the plunger accommodating hole, adjacent the bottom end thereof. Four crimped protrusions 542c are provided at four locations on the outer circumference of the ball guide at the inlet side thereof. The retainer 543 is provided with holes 543a for receiving the protrusions 542a. An oil communication hole 543b is provided in the retainer 543. A valve seat 544a is provided on the ball seat 544 for abutting engagement with the check ball 541, and the ball seat is held in place by the four crimped protrusions 542c. The two protrusions 542a, which hold the retainer to the ball guide, are inserted into holes 543a of the retainer 543, and then crimped in order to fix the retainer to the ball guide.
Several problems are encountered in the assembly and use of the check valve structure of FIGS. 6-7 (C). The parts of the protrusions 542a that extend through holes 543a are liable to become chipped. The width of the crimped portions of the protrusions 542a is limited. The end of the plunger-biasing spring 530 is not fixed relative to the disc-shaped retainer 543. Furthermore, it is difficult to position the holes 543a in the retainer 543 so that they receive protrusions 542a of the ball guide 542. Finally, since the ball seat 544 is fixed by the four crimped protrusions 542c provided on the outer circumference of the ball guide 542, the protrusions 542c are liable to become chipped. The widths of these protrusions are also limited.
The principal object of this invention is to provide a hydraulic tensioner which can be assembled easily, and in which the check valve unit can be readily assembled and incorporated into the tensioner easily and securely.